1. Field of the Invention
The present invention relates to an optical network unit and an optical line terminal. More particularly, the present invention relates to an optical network unit coupled to an optical access network system, which receives a data stream and dechurns information contained in the received data stream by using a churning key. The invention also relates to an optical line terminal coupled to the optical access network system, which transmits a data stream containing information that is churned with a churning key.
2. Description of the Related Art
Increasing numbers of telecommunication and multimedia services are provided today to serve for the growing market needs, including video on demand, cable TV, and high-speed access to computer networks. Those high-bandwidth services, however, should not raise the cost to subscribers. Here, optical access network systems are expected to play an essential role, connecting subscriber premises to the nearest local office exchange through fiber optic cables, rather than conventional metallic wires.
One of such systems is called the Passive Double Star (PDS), which enables a plurality of subscribers to share a single optical fiber line by using star couplers. Particularly in Europe, the Passive Optical Network (PON) system, synonymous with PDS, is of great interest as an enabling technology for the Fiber To The Home (FTTH) services. In the scenarios toward FTTH, the access network has to provide guaranteed bandwidths and quality of services to meet the requirements for real-time voice and video communication. To this end, the Full Service Access Networks (FSAN) initiative has a central role in the development of ATM-PON systems based on the Asynchronous Transfer Mode (ATM) technologies. The FSAN is an organization formed by major telephone companies to promote worldwide optical network businesses.
FIG. 25 shows a typical structure of an ATM-PON system. Optical network units (ONUs) 101a to 101n are deployed in subscriber premises 100a to 100n, while an optical line terminal (OLT) 201 is placed in a local office 200. Fiber optic cables and a star coupler 300 interconnect those ONUs 101a to 101n and OLT 201 in a point-to-multipoint fashion. In the subscriber premises 100a to 100n, telephone equipment and/or CATV equipment is coupled to the ONUs 101a to 101n. Connected to the OLT 201 in the local office 200 is ATM and ISDN switching equipment 202.
In the downstream direction, the local office 200 broadcasts data (i.e., downstream cells) toward the subscriber premises 100a to 100n over a single optical fiber cable. The star coupler 300 splits the optical signal into a plurality of signals in a tree and branch form, so as to deliver the information to individual subscribers' ONUs. In the upstream direction, ATM cells are transmitted from the subscriber premises 100a to 100n toward the local office 200 over the same branch cables. The star coupler 300 consolidates them into a single optical signal for delivery to the local office 200 over a single fiber cable.
As described above, the ATM-PON systems are ATM-based, optically-coupled access networks which provide point-to-multipoint (1:n) connections between a local office and a plurality of customers through the use of star couplers 300. The ITU-T Recommendation G.983.1 is one of the relevant international standard specifications for such PON-based broadband optical access systems. This G.983.1 includes description of a data encryption function termed “churning” to offer a protection capability for data confidentiality purposes. This function is mandatory because, in a PON system, the OLT always physically broadcasts information downstream, but only one ONU at a time can decode the information. More specifically, in the system of FIG. 25, the OLT 201 first sends a certain downstream message to request each ONU (e.g., ONU 101a) to provide its churning key. In response to this request, the ONU 101a generates a churning key and sends it back to the OLT 201. With the received churning key, the OLT 201 encrypts, or churns, downstream cells before sending them out to the ONU 101a. This data churning operation for downstream cells are performed on an individual virtual path (VP) basis. The OLT 201 notifies the ONU 101a of which virtual path is churned or not, by sending a special downstream message indicating the virtual path identifier (VPI) of a particular path that is churned or not churned. This information is referred to herein as “churning parameters.”
In summary, all ONUs in an ATM-PON system have their respective churning keys, and the churning of downstream information can be enabled or disabled separately for each VPI. The OLT sends downstream messages to notify each ONU of churning parameters before sending downstream cells. When data is received through a churned VP, the destination ONU decodes the data with its own churning key.
One problem with the above-described conventional system is that the ITU-T Recommendation G.983.1 lacks definitions for some specifics of the data dechurning functions to be used in ATM-PON systems. Take churning parameters stored in the ONU 101a for example. While those parameters are supplied from the OLT 201, the Recommendation G.983.1 does not stipulate when to activate the supplied parameters. This means that the data dechurning operation in the ONU 101a could be shifted in time, relative to the data churning operation in the OLT 201, and the time shift may grow up to such a critical level where the ONU 101a cannot decode the churned data correctly.
Another problem with the conventional ONUs is that they have to reload churning parameters from the OLT when they are rebooted after a power shutdown. This parameter reloading is a time-consuming process, while it is mandatory because the shutdown of ONUs clears out their stored churning parameters.
As seen from the above, conventional ATM-PON systems are still immature in terms of data churning techniques. It is therefore necessary to establish improved communication control algorithms in order to make ATM-PON systems truly practical.